The best alternative to Spline is Blender, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Spline and five of them are so hopefully you can find a suitable replacement. Other interesting alternatives to Spline are Cinema 4D, Hology Engine, Assetforge and BlenderCompat.
A comprehensive open-source 3D software suite for modeling, UV unwrapping, texturing, rigging, and animating. It supports renderings, simulations including fluid and cloth dynamics, complex compositing, video editing, and Python scripting, running across all major operating systems.
CINEMA 4D is a commercial, cross-platform, high-end 3-D graphics application, produced by MAXON Computer GmbH of Friedrichsdorf, Germany. Noted for its flexible interface and ease of use, it is capable of procedural and polygonal/subd modelling, animating, lighting, texturing...
3D game engine that uses web technologies. Perfect for those who already know TypeScript and want to collaborate with designers and artists to create 3D experiences.
Asset Forge allows anyone to create 3D models and 2D sprites using building blocks. Finished assets are compatible with most available game engines.
Compatibility DLL for running Blender 3.x on Windows 7 and a patch to modify the Blender source to use it.
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Spline is specifically designed to put WebGL scenes into websites. Cinema 4D is a standalone desktop 3D modeling software.